Many proposals have been made to contact a liquid or a mixture of a liquid and a gas with solid particles. These proposals were made, for example, in Japanese Patent Publications Nos. 26105/67 and 29162/72. Japanese Patent Publication No. 26105/67 discloses that when solid particles are placed in a contacting vessel, and a fluid is introduced into the vessel from its lower parts, the layer of the solid particles expands from its initially packed state, and the solid particles make a substantial motion, behaving as if they were a liquid. The fluidized bed is formed usually depending upon the specific gravity of the particles, their size, the amount of the particles accomodated, and the flow rate of the fluid. Furthermore, Japanese Patent Publication No. 29162/72 contemplates the improvement of the contact between a fluid and solid particles by providing an inside cylindrical body within a contacting vessel and thus circulating the solid particles regularly.
Contacting of a fluid with solid particles utilizing fluidized beds as mentioned above is a superior expedient because it can ensure intimate and uniform contact. Furthermore, according to methods of this type, the solid particles can be exchanged while the contact between the fluid and the solid particles is continued. This is a characteristic not seen in fixed-bed methods. However, the fluidized state of solid particles is essentially dominated by the flow rate of the fluid since the fluidized bed method usually comprises introducing a fluid from the lower part of a contacting vessel, contacting the solid particles with the fluid while fluidizing the solid particles within the contact vessel, and then withdrawing the fluid from the vessel. If the flow rate of the fluid is too low, the solid particles cannot be fluidized uniformly, and therefore, the contact is non-uniform. Furthermore, when the flow rate of the fluid exceeds a certain limit, the stability of the fluidized bed is rapidly destroyed, and the solid particles flow out of the system by being carried on the fluid. Especially when the fluid contains the bubbles, the interfaces of the bubbles interfere with the solid particles, and the bubbles entrain the solid particles irrespective of the flow rate of the fluid, so that the fluidized bed becomes very unstable.
Generally, the smaller the size of solid particles, the larger is the area of contact, and the more intimate is the contact between the solid particles and a fluid, but there is a stronger tendency that the solid particles flow out of the contacting vessel. In such a case, measures are taken to feed the solid particles as a slurry in the fluid to be treated, withdrawing the solid particles as a slurry in the fluid, and allowing the slurry to stand in a separate vessel thereby to separate the solid particles from the fluid.